Volatilization induced by impacts recorded in Zn isotope composition of ureilites

Ureilites are ultramafic achondrites formed by igneous processes early in the history of the Solar System. Ureilites are heavily depleted in volatile elements, probably by evaporation at the surface of the ureilite parent body: however the moderately volatile element Zn is an exception and its abundance is similar to Cl. Volatilization can fractionate isotopes in a mass-dependent manner. Therefore, comparing the isotope composition of volatile elements in ureilites may better constrain the physical conditions during ureilite formation and the mechanism of origin of the variation of Zn content. For this study, we measured the Zn isotopic composition of 11 ureilites with a large range of Zn content. The elemental abundance of Zn is negatively correlated with delta Zn-66, which may reflect that isotopic fractionation occurred by evaporation during the heating event on the ureilite parent body. Simple Rayleigh distillation predicts isotopic fractionations much larger than what is actually observed, therefore, such a model cannot account for the observed Zn isotope fractionation in ureilites. We propose that the observed isotopic fractionation can be produced by evaporation in a diffusion-limited regime. In addition, the isotopic composition of Zn is shown to be correlated with the shock grade of the ureilites. This may indicate that the heating events responsible for the depletion in light isotopes in some ureilites were impacts. (C) 2010 Elsevier B.V. All rights reserved.